Degaussing circuits



Feb. 10, 1970 J. GERRITSEN E AL DEGAUSSING CIRCUITS Filed 001:. 31, 1966 INVENTORS EN ANTELBERG AGENT United States Patent 3,495,136 DEGAUSSING CIRCUITS Jan Gerritsen and Adrianus Hubertus Kantelberg, Em-

masingel, Eindhoven, Netherlands, assignors, by mesne assignments, to US. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Oct. 31, 1966, Ser. No. 590,624 Claims priority, application Netherlands, Nov. 3, 1965, 6514206 Int. Cl. H01h 47/00; H01f 13/00 US. 317-157 .5 7 Claims ABSTRACT OF THE DISCLOSURE The present invention realtes to apparatus for degaussing television receivers. Apparatus of this type generally comprise a degaussing circuit including a coil through which an alternating current flows which decreases in amplitude in the loaded condition. The circuit also includes a supply resistor through which the coil is energized from an alternating voltage source, said supply resistor exhibiting an increasing resistance value in the loaded condition.

For degaussing magnetized material it is common practice to produce a magnetic field with alternately opposite directions in the material. The material is first magnetically saturated after which the amplitude of the magnetic field decreases to zero. It is known to produce the desired magnetic field with a coil by supplying the coil from an alternating voltage source through a supply resistor whose resistance increases in the loaded condition. This supply resistor may be of the type having a large positive temperature coetficient. It is found in practice that with a system supplied directly from the supply lines, the final value of the current in the coil for the greatest resistance value of the supply resistor produces a remanence fiel-d which may be disturbing. This disturbing remanence field occurs in particular in the demagnetization of the ferromagnetic parts magnetized by the terrestial magnetism, for example, the screening cap and the shadow mask of shadow mask colour television tubes. In this case the problem is solved by switching off the output current in the coil by an automatically operated switch when the amplitude of the output current in the coil is sufliciently small, since otherwise an undesired remanence is retained in the ferromagnetic parts to be degaussed.

The invention is based on the insight that, in particular in shadow mask colour television tubes, in the final condition of the demagnetisation the voltage across the degaussing coil must be made so small that a negligible magnetic remanence field is produced by the current in the coil. In this case we no longer require an expensive automatically operated switch.

For that purpose the circuit arrangement according to the invention is characterized in that, in addition to the supply resistor, a second resistor is connected in series with the coil, the resistance value of which second resistor also increases in the loaded condition, and a third resistor is connected parallel to the series arrangement of the coil and the second resistor, the quotient of the resistance values of the third resistor and the supply resistor decreasing in the loaded condition.

3,495,136 Patented Feb. 10, 1970 In order that the invention may readily be carried into effect, a few examples thereof will now be described in greater detail, by way of example, with reference to the accompanying figures, in which:

FIGURE 1 shows an embodiment of the degaussing circuit in a colour television receiver,

FIGURE 2 shows a second embodiment, and

FIGURE 3 shows a third embodiment.

in FIGURE 1, the degaussing coil 1 is supplied by an alternating voltage source, for example, the supply lines 2, through a supply resistor 3. The coil 1 may be divided into several fractional coils. The supply resistor 3 is constructed from a seriesto-parallel circuit composed of the filaments 12 of the tubes in a colour television receiver. When the supply switch 4 of the colour television receiver is actuated, the low resistance value of the filament circuit 3, associated with the cold condition, will rapidly increase to the higher resistance value which is associated with the higher temperature occurring in the operating condition of the colour television receiver. Alternatively, an additional resistor having a positive temperature coefficient (PTC-resistor) may be arranged in the receiver so that this resistor can take over the operation of the filament circuit 3 shown in FIGURE 1.

According to the principle of the invention, a resistor 5 having a positive temperature coefficient (PTC-resistor) is connected in series with the coil 1 and a resistor 6 having a negative temperature coefiicient (NTC-resistor) is connected parallel to the series arrangement of the coil 1 and the FTC-resistor 5. The resistors 5 and 6, which are each included in a different branch of a parallel circuit, are thermally coupled by bringing them in intimate thermal contact with one another.

Before actuating the supply switch 4, all the resistors are at ambient temperature and have such values that the NTC-resistor 6 is large relative to the small resistors 3 and 5. The initial current occurring immediately after actuation of the switch 4 flows substantially through the series arrangement of the supply resistor 3, the coil 1, and the FTC-resistor 5. The amplitude of this initial current is large as a result of the small initial values of the resistors 3 and 5. This large initial current directly heats the supply resistor 3 and the PFC-resistor 5 so that the resistance of said resistors increases. The NTC-resistor 6 is directly heated by the small current which initially flows through it and is indirectly heated by means of the thermal coupling with resistor 5, through which a large initial current is flowing. As a result, the resistance value of the NTC-resistor 6 will rapidly decrease so that the current through said resistor can increase. The resulting larger direct heating will produce a further decrease of its resistance value. This means that as a result of the thermal coupling between the resistors 5 and 6, a cumulative effect is obtained so that, as a result of the rapid de crease of the resistance value of the NTC-resistor 6 and the rapid increase of the resistance value of the PTC- resistor 5, the current flowing through the parallel arrangement which initially flows substantially entirely through the coil 1, will then choose its way through the decreased NTC-resistor 6.

In the operating condition of the colour television receiver, the resistance value of the NTC-resistor 6 will be small relative to that of the supply resistor 3 and the PTC-resistor 5. As a result of the voltage division of the supply voltage between the supply resistor 3 which is now large and the small resistance of the parallel circuit which is nearly entirely determined by the decreased NTC- resistor 6, a small voltage is set up across the parallel circuit of the elements 1, 5 and 6. In addition, the resistance value of the FTC-resistor 5 has increased so that only a negligible current will flow through the coil 1,

through which a negligible remanence field remains. The thermal coupling of the electrically parallel arranged PTC and NTC-resistors 5 and 6, respectively, which as such are slow, produces, as a result of the combination of the two, a rapid variation in the current distribution across the parallel branches. With the above described method the switching-on current pulse in the filament circuit is limited while the heating time of the receiver is not lengthened appreciably. In the operating condition of the colour television receiver, when the current through the filament circuit 3 flows nearly entirely through the NTC-resistor 6, the FTC-resistor 5 is indirectly heated by means of the thermal coupling, so that the current through the coil 1 and the FTC-resistor 5 decreases to very small values.

It is to be noted that the circuit arrangement shown 1n FIGURE 1 requires no extra components. In the known circuit the FTC-resistor in series with the coil was directly connected to the supply lines through the power switch. The filament circuit with a series connected NTC- reslstor as a current limiter constituted a separate circuit. By combining the two circuits in the manner shown in FIGURE 1, the above described advantages are achieved substantially without additional costs. It should only be ensured that the collective resistance value of the coil 1 and the FTC-resistor 5 in the cold condition is sufiiciently large so that the initial current through the filament circuit 3 cannot damage any of the filaments.

In addition it is to be noted that replacing a PTC- r'esistor (as used in the known circuit) by the filament circuit 3 has the advantage that the life of the circuit arrangement is lengthened. In fact it was found that a FTC-resistor, across which during operation substantially the whole supply voltage of, for example, 220 v., is set up, is rapidly worn out. This is by no means the case with a filament circuit. In addition, in a receiver with the known circuit, a switching-on current pulse occurs which consists of two components, namely one through the filament circuit and one through the degaussing circuit. This extra load on the supply lines by the degaussing circuit does not take place in the circuit arrangement according to the invention as described.

It will also be clear that, if the colour television receiver comprises several filament circuits, at least one of these circuits may serve as a supply resistor 3.

In FIGURE 2, the reference numerals already used in FIGURE 1 denote the same elements. A voltage-dependent resistor 7 (VDR) is connected in series with the coil 1. The VDR is a device which has a small resistance when the voltage impressed across the resistor is high and a large resistance when the voltage across the resistor is low. The resistor 8 is a normal resistor which maintains a substantially constant resistance value during operation. The resistors are proportioned so that when they are supplied from the supply lines, and with a cold filament circuit, the constant resistor 8 is large relative to the supply resistor 3 and the VDR. In the operating condition of the colour television receiver, the constant resistor 8 is small relative to the increased resistance of resistors 3 and 7. The switching-on current pulse is determined by the series arrangement of the supply resistor 3, the coil 1, and the VDR 7. This current heats the filament circuit 3 so that the resistance value of the resistor 3 increases. Then a voltage division occurs of the supply voltage between the series arrangement of the resistors 3 and 8, in which the greater part of the supply voltage is across the resistor 3, so that as a result of the smaller voltage across the parallel circuit the resistance value of VDR 7 increases. In the operating condition of the colour television receiver, the supply current of the receiver will flow for the greater part through the resistor 8 across which a small voltage is set up, so that the resistance of VDR 7 becomes so large that the residual current flowing through the coil 1 produces a negligible magnetic field. This circuit arrangement is very cheap since, besides the degaussing coil 1, only a normal resistor 8, which is suitable for the supply voltage, and the VDR 7 are required. This circuit is even cheaper than that shown in FIGURE 1, since, when the prices of the VDR 7 and the PTC-resistor 5 are the same, the more expensive NTC-resistor 6 is replaced by a cheaper ohmic resistor 8.

Both of the circuits described produce satisfactory degaussing of the ferromagnetic parts in a colour television receiver when the receiver is switched on. If, for some reason or other, degaussing is also desirable during operation, this is not possible with the circuit arrangement shown in FIGURES 1 and 2. In fact, for a ready operation the filament circuit 3 must be at ambient temperature. So the user would have to switch oit the receiver and allow the filament circuit to cool for some time, which is undesirable. A solution to this problem is described with reference to FIGURE 3. i

In FIGURE 3, the filament circuit 3 of the receiver used in FIGURE 2 is replaced by two parallel-arranged FTC-resistors 9 and 10, one of which can be connected in the degaussing circuit by a separate two-way switch 11. The switch 11 is of a very simple construction. The PTC- resistor connected in the degaussing circuit, for example, the resistor 9, operates the same way as the filament circuit 3 discussed with reference to FIGURE 2 and of degaussing occurs in the manner described with reference to FIGURE 2. If it is desirable to degauss once again during operation of the receiver, the switch 11 is switched to the cold FTC-resistor 10. After a short period of time the PTC-resistor 9 will cool down so that with this circuit arrangement it is possible to degauss repeatedly Without having to interrupt the voltage supply to the colour television receiver. For the above described circuit it is required that, with the exception of the PTC- resistors 9 and 10, no further temperature-dependent ele ments are included in the degaussing circuit.

It is clear that the degaussing circuits described are by no means to be used only for degaussing the ferromagnetic parts in a shadow mask colour television receiver. For toolsto be used in the demagnetized condition, for example, screw drivers, tongs, scissors, and so on, in repairs or routine tests of, for example, magnetic recorders and/or playback apparatus, a demagnetizing circuit as described with reference to FIGURE 3 can readily be used. This circuit arrangement may also be used for the so-called fast erasing of magnetic tape in which a magnetized tape can be demagnetized in one pass.

What is claimed is:

1. A degaussing circuit for a body of magnetizable material comprising, a neutralizing coil adapted to be placed adjacent said magnetizable body, a source of alternating voltage, a supply resistor having a. positive temperature coefficient, a second resistor having a positive temperature coefiicient, means connecting said supply resistor, said coil, and said second resistor in series across said voltage source, a third resistor having a negative temperature coefiicient connected in parallel across the series combination of the coil and the second resistor, said supply resistor and said second resistor exhibiting an increase in resistance from the moment they are initially energized and for a given period of time thereafter so that the quotient of the resistance values of the third resistor and the supply resistor decreases during said time period, whereby the AC current flowing in said coil decreases in amplitude.

2. A circuit as claimed in claim 1 wherein said second and third resistors are thermally coupled.

3. A circuit as claimed in claim 2 wherein the supply resistor comprises at least a portion of the filament circuit of an electric device.

4. A degaussing circuit for a body of magnetizable material comprising, a neutralizing coil adapted to be placed adjacent said magnetizable body, a source of alternating voltage, a supply resistor comprising at least one filament circuit of a television receiver, a second resistor, means connecting said supply resistor, said coil, and said second resistor in series across said voltage source, a third resistor connected in parallel across the series combination of the coil and the second resistor, said supply resistor and said second resistor exhibiting an increase in resistance from the moment they are initially energized and for a given period of time thereafter so that the quotient of the resistance values of the third resistor and the supply resistor decreases during said time period, whereby the AC current flowing in said coil decreases in amplitude.

5. A degaussing circuit for a body of magnetizable material comprising, a neutralizing coil adapted to be placed adjacent said magnetizable body, a source of alternatingvoltage, a supply resistor divided into at least two resistors each having a positive temperature coefficient, a second resistor, a switch having two positions, mean connecting said switch in circuit with said supply resistors, said coil, and said second resistors, so that in one position of the switch one supply resistor is connected in series with said coil and said second resistor across said voltage source and in the second switch position the other supply resistor is connected in series with said coil and said second resistor across said voltage source, and a third resistor connected in parallel across the series combination of the coil and the second resistor, said supply resistors and said second resistor exhibiting an increase in resistance from the moment they are initially energized and for a given period of time thereafter so that the quotient of the resistance values of the third resistor and the supply resistors decreases during said time period, whereby the AC current flowing in said coil decreases in amplitude.

6. A degaussing circuit for a body of magnetizable material comprising, a neutralizing coil adapted to be placed adjacent said magnetizable body, a source of alternating voltage, a supply resistor of the type having a positive temperature coeflicient, a second resistor of the type that is voltage-dependent, means connecting said supply resistor, said coil, and said second resistor in series across said voltage source, a third resistor that exhibits a substantially constant resistance over the operating range of the circuit and connected in parallel across the series combination of the coil and the second resistor, said supply resistor and said second resistor exhibiting an increase in resistance from the moment they are initially energized and for a given period of time thereafter so that the quotient of the resistance values of the third resistor and the supply resistor decreases during said time period, whereby the AC current flowing in said coil decreases in amplitude.

7. A degaussing circuit for a body of magnetizable material comprising, a neutralizing coil adapted to be placed adjacent said magnetizable body, a source of alternating voltage, a supply resistor comprising at least a portion of the filament circuit of a television receiver, a second resistor of the type that is voltage-dependent, means connecting said supply resistor, said coil, and said second resistor in series across said voltage source, a third resistor that exhibits a substantially constant resistance value during operation and connected in parallel across the series combination of the coil and the second resistor, said supply resistor and said second resistor exhibiting an increase in resistance from the moment they are initially energized and for a given period of time thereafter so that the quotient of the resistance values of the third resis tor and the supply resistor decreases during said time period, whereby the AC current flowing in said coil decreases in amplitude.

References Cited UNITED STATES PATENTS 3,322,998 5/1967 Norley 315-8 US. Cl. X.R. 315-8 

